Thermally coupled distillation and the concept of carrying out such a distillation in a single column with a dividing wall separating the feed side from the side product side have been known for a long time (see e.g., U.S. Pat. No. 2,295,256 to Brugma, U.S. Pat. No. 2,471,134 to Wright, and the paper by Triantafyllou and Smith [Design and optimization of fully thermally coupled distillation columns. Trans I. Chem. E, Vol 70, Part A (1992), pages 118-132]).
Divided wall columns typically have a configuration in which a vertical baffle in the middle section of the column separates the feed point from the point at which the intermediate boiling point product is withdrawn. In this way, the quantities of both light and heavy components in the feed reaching the side draw may be controlled at any specification (which is traditionally achieved in two conventional distillation steps). This allows for various advantages, including significant reduction of reboiler and condenser duties (e.g. up to 30% as compared to a two-column arrangement), and further reduces capital cost by reducing the number of required components.
Despite these and other advantages of employing a divided wall column for separation of at least ternary feeds, several aspects of these columns remain problematic. Among other things, identification of suitable properties that will adequately correlate with the desired separation on the feed side of the partition has been difficult for many years. For example, effective control and operation of a divided wall column typically requires adequate separation of the lights from the heavies on the feed side. If excess heavies rise to the top of the partition on the feed side, the intermediate product will be contaminated with heavies. If excess lights descend to the bottom of the partition on the feed side, the intermediate product will be contaminated with lights. Separation of the intermediate component is less problematic, since the intermediate component can distribute either way (i.e., to the top or bottom of the feed side column).
One method of achieving adequate separation of the lights from the heavies on the feed side includes use of temperature indicators/sensors to detect movements of lights towards the bottom of the feed side of the partition (Lights will be indicated by a drop in temperature). While temperature measurement is conceptually viable, temperature measurement is frequently limited to a scenario in which the concentration of lights has a significant effect on tray temperatures below the feed. Unfortunately, it is relatively common that the tray temperature is insensitive to the presence of lights, especially when the concentration of lights below the feed is relatively small.
To circumvent at least some of the problems associated with temperature indicators/sensors, analyzers may be used to detect the composition of the feed at a particular position in the divided wall column on the feed side. However, analyzers tend to be expensive, unreliable (at least in some cases), and frequently exhibit significant lag times, which adds further problems in tower control.
Although various methods and configurations for operation and monitoring of divided wall columns are known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide improved methods and configurations for operation and monitoring of divided wall columns.